1. Field of the Invention
This invention relates to a distance detector used in a laser beam machine to detect a focal position of a laser beam or a height of a nozzle.
2. Description of Related Art
In a laser beam machining, several machining conditions are properly set in general, depending on a material or a plate thickness of a workpiece in general. For example, laser output, pulse frequency, assist gas pressure, focal position, or nozzle height should be set and kept appropriately in the machining. Particularly, the focal position and nozzle height are delicate conditions in the machining, and they should be controlled precisely.
Moreover, the laser beam machining is a stroke or linear machining. When the machining is carried out while the laser beam relatively scanning the workpiece, it is necessary to sense a workpiece position and adjust the focal position and nozzle height.
In order to detect the focal position and nozzle height, a distance detector is fitted on a leading end of the nozzle. However, the focus lens and the nozzle are usually mounted on the same frame of the laser beam machine. Therefore, the detector does not individually sense the focal position and nozzle height relative to the workpiece. In general, the distance detector measures the distance between the nozzle end and the workpiece to determine the nozzle height, thereby detecting the position of the focusing lens and a relative position of the nozzle end.
The laser beam machining has increased uses in recent years. Accordingly, there are increased needs for machining metal plate workpieces such as aluminum or stainless steel for decorative use. Therefore, a non-contact type detector is preferably used as the distance detector for preventing scratch by contact. Particularly, a distance detector utilizing a capacity is used in many cases. In addition, there is a request for a distance detector having high responsibility so that the laser beam machining is performed at high speed. In the present situation, the capacitance type distance detector is preferred to a mechanical type.
It is common that such capacitance type distance detector has a sensing electrode disposed at the nozzle end and electrically applies an alternating signal (voltage or current) to the sensing electrode. Then, the alternating signal changes according to capacitance between the sensing electrode and the workpiece. Such changing alternating signal is sensed to determine the capacitance between the electrode and workpiece, so that the alternating signal is transformed into a dc signal that corresponds to the distance between the electrode and workpiece. The dc signal is supplied to a control device thereafter. The control device controls the nozzle height or the distance between the electrode and workpiece in accordance with the dc signal so that it is kept at a desired value.
Specifically, the laser beam machine concentrates the laser beam into a beam spot thereby to radiate it on the workpiece. Then, the material is melted and sublimated to be scattered instantaneously. Thus, a removing machining or mainly cutting is conducted. Therefore, a condenser lens or condenser mirror is provided to condense the laser beam, and in general, a machining nozzle is furnished to jet an assist gas to blow away the melted or sublimated material.
Moreover, it is common to use a sinusoidal signal as the alternating signal for determining the capacitance between the sensing electrode and workpiece. In this case, a voltage of the sensing electrode is detected as a signal corresponding to the capacitance. An amplitude V of the voltage of the sensing electrode is give by the following expression, where a sine wave alternating current of frequency f and amplitude i is applied to the sensing electrode. EQU V=2.pi.fCi
The detected voltage of the sensing electrode becomes a sinusoidal wave of the amplitude V and frequency f. Therefore, such sinusoidal wave is usually rectified and transformed into a dc signal proportional to the amplitude V.
In general, the nozzle end of the laser beam machine should be small in view of operationability. Then, the sensing electrode attached thereto should be small-sized, accordingly. Therefore, the capacitance between the electrode and workpiece is very small, normally 1 pF or less. Moreover, the frequency of the alternating signal is determined by a response frequency regulated by a performance of the laser beam machine. Normally, a frequency of 10 kHz or more is used.
Since the sensed voltage is transformed by the weak capacitance C, it is very easily affected by a disturbance. Accordingly, the dc signal after rectification is further passed through a low pass filter so that a high frequency component of the disturbance is cut off.
A specific circuit of the conventional distance detector described above is exemplified hereunder.
FIG. 14 shows an overall structure of a first conventional distance detector for laser beam machine. FIG. 15 schematically shows such distance detector.
Referring to FIG. 14, a constant alternating current source 7 generates an output of i=Isin2.pi.ft wherein I is an amplitude of a current, t is a time and f is a frequency. An annular sensing electrode 1 is arranged at a leading end of a nozzle 3 and placed around an area where a laser beam 4 passes through. The current source 7 supplies a constant current i to the sensing electrode 1. The constant current i is transformed by the capacitance C into a voltage as an alternating signal. Such voltage is detected by a voltage detecting circuit 8. Then, the Alternating current is transformed into a dc signal in a rectifying circuit 9. The dc signal contains a high frequency component. Therefore, such unnecessary high frequency component is cut off at a low pass filter 10. Thus, a distance detecting output Vo is obtained.
As mentioned above, the sensed voltage of the sensing electrode 1 serves for measuring a focal position 6 and a nozzle height in relation to a workpiece 2. Namely, the distance detector measures the distance from the end of the nozzle 3 to the workpiece 2, thereby detecting the nozzle height. Then, the detector determines a position of a not-shown condenser lens and a relative position of the nozzle end.
The alternating signal source applied to the sensing electrode 1 may be not only the constant sine-current source, but also a constant sine voltage source. FIG. 16 shows such example or an overall structure of a second conventional distance detector of a laser beam machine. In FIG. 16, the same characters and numerals as those of the first conventional art indicate the same or corresponding elements as those of the first conventional art.
Referring to FIG. 16, the constant alternating voltage source 7A generates an output of v=Vsin2.pi.ft. As in the first conventional art, the sensing electrode 1 is disposed at the end of the nozzle 3. A current io is supplied to from the voltage source 7A to the sensing electrode 1. Then, the current io is fed to the capacitance C between the electrode 1 and workpiece 2. A current transformer 13 detects such current io. A current detecting circuit 14 transforms the current io from the transformer 13 into a voltage proportional to the current value. The rectifying circuit 9 transforms an output as an alternating signal from the detecting circuit 14 into a dc signal. Such dc signal contains a high frequency component. Therefore, the low pass filter 10 cuts off the unnecessary high frequency component, thereby providing the distance detecting output Vo.
As mentioned above, the sensed voltage of the sensing electrode 1 serves for measuring the focal position 6 and the nozzle height in relation to the workpiece 2, as in the first conventional art. Then, the nozzle height is detected, so that the position of the condenser lens and the relative position of the nozzle end are determined.
In this conventional art, the sensed current io flowing into the electrode 1 is: EQU io=V/2.pi.fC
where an amplitude of applied voltage is V, frequency is f and unknown capacitance is C.
In these conventional distance detectors, the low pass filter 10 gets rid of only a part of frequency components included in the disturbance, which is caused by spatter and plasma generated during machining. Consequently, error factor in the distance detector cannot be completely removed.
Particularly, the needs for machining the aluminum, stainless steel or the like increase, as mentioned above. These materials tend to generate plasma or spatter at a laser beam irradiated portion in the laser machining. The capacitance type distance detector constructed as above is subject to the influence of the plasma or spatter, which are the factors in malfunction of the detector.
Such malfunction of the detector causes not only defective laser machining but also collision of the workpiece 2 and he nozzle 3 at worst, thereby damaging the workpiece 2 or laser beam machine itself.
On the other hand, an automatic laser beam machining is introduced due to a demand for saving labor in recent years. Accordingly, reliability in the machining has become more important.